docs: def append(self, clause):
          """
              Add one more clause to CNF formula. This method additionally
              updates the number of variables, i.e. variable ``self.nv``, used in
              the formula.
  
              :param clause: a new clause to add.
              :type clause: list(int)
  
              .. code-block:: python
  
                  >>> from pysat.formula import CNF
                  >>> cnf = CNF(from_clauses=[[-1, 2], [3]])
                  >>> cnf.append([-3, 4])
                  >>> print cnf.clauses
                  [[-1, 2], [3], [-3, 4]]
          """
  
          self.nv = max([abs(l) for l in clause] + [self.nv])
          self.clauses.append(clause)